The war made keen cricketer and poet Siegfried Sassoon a “20th century icon of suffering and sacrifice”.

This is the view of academic Jean Moorcroft Wilson whose latest book is billed as the definitive single-volume biography of Sassoon.

Siegfried Sassoon: Soldier, Poet, Lover, Friend documents the Great War poet’s life from birth in 1886 to his death in 1967 as observed through the historian’s lens.

But Ms Wilson, of Mornington Place, Mornington Crescent, is under no illusion that it was the four years between 1914 and 1918 that defined the life and legacy of Sassoon.

“The war made Sassoon, I think that’s what made it so difficult for him,” said Ms Wilson. “He looked back and felt like a booby-trapped idealist. He felt used by the pacifists.”

It is his vivid wartime verse, depicting the horrors of the trenches among other things, which has immortalised Sassoon, yet he is also remembered as one of the most vocal opponents of the war within the British Army at the time.

His encounters with aristocrat Lady Ottoline Morrell and philosopher Bertrand Russell while he recuperated from war injuries between 1916 and 1917 turned Sassoon toward the doctrine of pacifism.

He subsequently wrote a protest letter against the war that was read out in the House of Commons and made headlines in newspapers across the world, risking a court martial, prison and, at worst, death by firing squad.

The poet avoided all these fates and went onto become a spokesman for pacifism throughout the 1920s and into the 1930s before the advance of Hitler in Europe persuaded him to abandon his pacifist outlook.

However, he could never shake off the shackles of being an anti-war champion in his youth and it became an indelible hallmark of the poet’s life story.

Sassoon was born to a Jewish father and an Anglo-Catholic mother in Matfield, Kent. He went on to attend prestigious public school Marlborough College, in Wiltshire, before winning a place to study history at Cambridge University’s Clare College.

He left university without a degree and spent the next few years hunting, playing cricket and writing verse until the threat of the First World War was realised and he decided to join the army.

Ms Wilson, an English Literature lecturer at the University of London and considered the world’s foremost expert on Sassoon, described the poet at this time in his life as a “patriotic young public schoolboy who wanted to fight for his country”.

“War was also very exciting and in addition he’d run out of money living the life of the gentleman,” she said. “With the war he got a subject for the poetry and he got an aim in life.”

Sassoon first joined the Sussex Yeomanry regiment of the British Army but quickly became disillusioned by the unit, which was stationed in the UK and contained mainly married family men who Sassoon had little in common with.

The young Sassoon wanted to see frontline action and joined the Royal Welch Fusiliers as a trooper.

“He was what we might call jingoistic, he wanted to fight,” said Ms Wilson. “He wanted to show them what the British were made of.”

Even with the death of his brother in Gallipoli in 1915, Sassoon remained passionately committed to the war cause and was posted to France in the same month of his brother’s passing.

But on the frontline, the reality of war quickly hit home to Sassoon and his attitude began to change.

“It wasn’t until he was in France watching men having to deal with the trenches, the rain, the rats, the terrible weight of the equipment, [that his mind changed],” said Ms Wilson. “It was a very shocking introduction to him.”

Ms Wilson believes Sassoon’s homosexuality, which he kept a secret from his fellow soldiers during the war, heightened the anguish he felt on the battlefield.

“The fact he was so deeply attracted to the men, I suppose he cared more when the men were killed,” she explained. “He thought it was outrageous to be involving men in this kind of danger. All of these factors made him realise that war was not glorious.

“Less than a month after getting out there he writes the first draft of a poem named The Redeemer. He sees the men as Christ-like figures lying down their lives for the country.”

Despite his growing distaste for the ethics of the war, Sassoon was revered as a fearless soldier, gaining the nickname “Mad Jack”, and ultimately winning the Military Cross for his bravery.

“He was incredibly brave, some would say foolhardy,” said Ms Wilson. “They used to call him Mad Jack because he would always volunteer to go out into No Man’s Land.

“He would actually enjoy it. He would lie on his back and laugh at the exhilaration of it all.”

Sassoon witnessed and documented the Battle of the Somme in 1916, one of humanity’s bloodiest battles.

“He is in reserve so he watches the whole battle from the reserve trench,” said Ms Wilson. “He watches the sun rise then he sees these men come out the trenches and walk towards the enemy and get mowed down. It’s a tremendously graphic description.”

The poet’s next taste of action is the Battle of Aras in April 1917 during which he is shot in the neck, narrowly avoiding death, and is sent home.

It is during this period of convalescence that he encounters the pacifists and writes his letter of protest.

Terrified by the potential consequences of his public defiance, Sassoon’s commanding officers send him to Craiglockhart War Hospital in Edinburgh where he is treated for “shell shock”.

At the hospital, he met fellow poet and solider Wilfred Owen, who was also being treated for shell shock, and the pair struck up a friendship.

“Owen’s war poem Anthem for Doomed Youth was slightly influenced by Sassoon,” said Ms Wilson. “Owen worshipped Sassoon when he met him, said he was writing poetry he wished he could write.”

The two met again in 1918 when Sassoon, who had previously returned to the frontline, came home after being shot in the head in the Battle of Lys.

Owen was preparing to be deployed himself when Sassoon returned and was urged by Sassoon not to go back to war, but to no avail.

On November 4, 1918, exactly a week before the Armistice, Owen was killed in action - a tragedy that could only have intensified Sassoon’s objections to the war.

“I don’t think Sassoon felt that they had made sufficient effort to resolve the conflict,” said Ms Wilson. “He said, ‘I entered it because I thought it was a just war and now I don’t think it is.’

“Sassoon was dissenting from a position of privilege. It’s alright dissenting when there’s nothing to lose. But he had a lot to lose. He was a gentleman, he had friends in the Establishment.

“To risk all that, I think he must have been very idealistic.”

Jean Moorcroft Wilson will be talking about her new book on September 14, as part of the Hampstead and Highgate Literary Festival 2014, at the London Jewish Cultural Centre (LJCC). For more information, visit www.hamhighlitfest.com